1. Field of the Invention
The present invention relates to an air-to-fuel ratio sensor, and more particularly, to an air-to-fuel ratio sensor having durability against thermal shock. The air-to-fuel ratio sensor also is capable of being operated easily by a driving circuit adopting a simple driving principle.
2. Description of Related Art
In an automobile engine, performance, fuel consumption and the amount of exhaust gases change according to the air-to-fuel (A/F) ratio. Thus, it is desirable to set an optimum A/F ratio according to driving conditions.
A majority of gasoline engines employ an oxygen sensor, which typically is called a lambda sensor. The lambda sensor enables gasoline engines to be operated in the stoichiometric point to maximize the efficiency of a 3-way catalyst. This sensor can detect fuel-lean or fuel-rich from the exhaust gas composition. This sensor shows abrupt change of signal, however, at the stoichiometric point. Accordingly, by using the lambda sensor, it is impossible to measure the degree of the A/F deviation from the stoichiometric point.
A lean-burn engine capable of operating at the fuel-lean condition for saving fuels, however, requires a wide-range A/F sensor capable of detecting how much rich or lean the fuel is, as well as whether the fuel is rich or not. Thus, there currently has been significant research into various wide-range A/F ratio sensors.
A wide-range A/F sensor enables detection of whether the fuel is rich or not, and is less dependent on temperature, representing a linear A/F ratio signal in the fuel lean condition.
FIG. 1 is a schematic sectional view showing a conventional wide range A/F ratio sensor. The A/F ratio sensor includes a pumping cell enabling movement of oxygen ions in a desired direction, a sensing cell enabling sensing of electromotive forces according to the difference in partial pressures of oxygen, a diffusion barrier 3 for controlling the diffusion of gas, a cavity 7 enclosed by the diffusion barrier 3, and a dense layer 6 for maintaining the partial pressure of oxygen contacting an air reference electrode 1b of the sensing cell to a predetermined level.
Here, the pumping cell includes a solid electrolyte 4 that is conductive to oxygen ions, and two electrodes 5a and 5b attached to both sides of the solid electrolyte 4 to move oxygen ions via the solid electrolyte 4.
Similarly, the sensing cell includes electrolyte 2 that is conductive to oxygen ions, and two electrodes 1a and 1b attached to both sides of the solid electrolyte 2. The electrodes 1a and 1b are used to measure internal electromotive forces generated by the difference in partial pressures of oxygen.
Also, the diffusion barrier 3 is formed between the sensing cell and the pumping cell, and the cavity 7 is formed between the sensing cell and the pumping cell, and is enclosed by the diffusion barrier 3. The diffusion barrier 3 acts to restrict the flow of the exhaust gas into the electrode 5a and is formed of a material that is not conductive to oxygen ions and electricity in order to prevent interference between the sensing cell and the pumping cell.
The A/F ratio sensor is based on the principle that both the amount and direction of the pumping current for maintaining the partial pressure of oxygen in the cavity 7 change as the partial pressure of oxygen contained in the exhaust changes. Referring to FIGS. 1, 2 and 3, the driving principles of the conventional A/F ratio sensor will be described in detail.
FIG. 2 is a graph illustrating the principle of sensing oxygen by a general limiting current sensor in the oxidation atmosphere. As shown in FIG. 2, current continuously increases together with an increase in the applied voltage in a section in which the amount of diffused oxygen is increased continuously. However, if the amount of pumped oxygen increases to a predetermined level or more, the diffusion of oxygen itself is limited, so that a predetermined pumping current flows. Here, the amount of oxygen diffused into a cathode is directly proportional to the concentration of oxygen outside the diffusion barrier. Thus, a pumping current proportional to the oxygen concentration can be obtained as a signal for a sensor.
Meanwhile, in a reduction atmosphere, oxygen is pumped to a diffusion barrier from the outside and then reacted with a reduction gas contained in the exhaust gas. Thus, the limiting current becomes proportional to the amount of the diffused reduction gas (FIG. 3).
As can be seen from FIGS. 2 and 3, in the conventional A/F ratio sensor operating by the above principle, the direction of the oxygen ion movement differs depending on whether the fuel is rich or lean. That is, in the fuel lean region, as a voltage Vp is applied to the pumping cell of the A/F ratio sensor, oxygen ions near the electrode 5a move toward the other electrode 5b via the solid electrolyte 4. In a state where the voltage is not applied to the pumping cell, there is scarcely any difference in the oxygen concentration between the electrodes 1a and 1b, so that the electromotive force of the sensing cell is almost zero. In order to maintain the equilibrium partial pressure of the oxygen in the cavity at approximately 10.sup.-10 atm, the electromotive force of the sensing cell should be at approximately 400-450 mV. The wide-range A/F ratio sensor controls the electromotive force of the sensing cell to approximately 400-450 mV through a feedback control of the pumping current by an electrical circuit. The current when the electromotive force of the sensing cell is 400-450 mV corresponds to the limiting currents with regard to oxygen. Therefore, the pumping current in this region is proportional to the equilibrium oxygen concentration in the exhaust gas.
On the other hand, the equilibrium partial pressure of oxygen is very low, i.e., approximately 10.sup.-20 atm in the fuel rich region, and the electromotive force of the cell generated by the difference in partial pressure of oxygen is very high, as much as approximately 900 mV. In order to increase the equilibrium oxygen pressure in the cavity 7 to approximately 10.sup.-10 atm, oxygen should be pumped into the internal electrode 5a from the external electrode 5b. Thus, as in the case where the fuel is lean, the pumping current for maintaining the voltage of the sensing cell at 450 mV through the feedback control is expressed by a signal. As the fuel is richer, there is more reducing gas and the equilibrium partial pressure of oxygen is low. In this case, the magnitude of the pumping current increases. On the electrode 5a, the reduction gas of the exhaust gas, such as carbon monoxide and hydrogen, reacts with oxygen provided via the solid electrolyte 4 to generate carbon dioxide and water.
As described above, in the conventional A/F ratio sensor, the direction of the oxygen ion movement is changed according to whether the fuel is rich or lean. Also, feedback control is required to maintain the equilibrium partial pressure of oxygen in the cavity to a predetermined level. Accordingly, in the conventional A/F ratio sensor, the driving circuit elements are complicated.
Also, in the conventional A/F ratio sensor, the electrolyte layers of the pumping cell and the sensing cell, the porous diffusion barriers and the dense layer for forming the air reference electrode are made of different materials. It is therefore difficult to control the difference of shrinkage during the co-firing of these components. Also, if the sensor is used for a long time at a high temperature, a conventional sensor having these various layers, each made of different materials, will likely become deteriorated by a cyclic thermal shock due to the different thermal expansion of the materials.